This invention relates to time delay circuits, and more particularly to an ultra wideband frequency dependent attenuator with a constant group delay capable of simulating the loss of a long delay line in a shorter length delay component.
Time delays are often realized in electronic systems with transmission lines of controlled length. The delay arises from the finite speed of electrical signals in the line. Different delays are often created by switching between a number of different delay lines having different lengths. Electronic systems employing delay lines include pulse generators, integrators, correlators, high speed samplers and sampling oscilloscopes, radar systems, phased array antennas and other communications systems.
A particular problem associated with switchable delay lines is that the longer the desired time delay (i.e., the longer the physical length of the delay line), the greater the loss becomes in the delay path. This is because of normal resistive losses in the metal and dielectric materials of the transmission line. The loss is almost always a function of frequency, with higher losses at higher frequencies being experienced. This characteristic of increasing loss with frequency is primarily the result of changing skin depth in the metal. When a switch is made between a short line and a longer delay line, the loss in the signal path changes. More specifically, the loss that will be experienced will be greater for the longer delay line.
The change in loss for different delays is a problem because an electronic system which is receiving signals passing through a plurality of different delay lines is often performing a summing action on the many signals, as in the case of a phased array antenna. The vector addition will be incorrect if the amplitudes of the signals vary significantly across different delay settings. Amplitude differences are also a problem in systems where a difference or other comparison between signals through different delay lines is required.
Any scheme to correct the loss occurring when a signal travels through a given delay line must also provide a constant time delay for all of the frequency components required for the system. If the constant time delay is not maintained, the electronic system which receives the signals passing through the time delay lines will have difficulty propagating pulses without distorting their shapes. This is because the high frequency components of the signals will suffer a phase change different from the low frequency components of the signals. The derivative of phase with respect to frequency is known as group delay. Extremely broadband communications systems including phased array antennas will have trouble meeting their specifications over the required bandwidth if the time delay is not constant for all frequencies of operation. This amounts to a requirement for constant group delay.
One approach to solving the above problem of different losses being experienced in a given signal depending upon the frequency of the given signal would be to eliminate the loss in the lines by employing a superconducting medium. Another approach would be to create a compensating attenuator circuit which can add loss to the shorter paths. These networks can be designed like a filter to have either increasing or decreasing loss at higher frequencies. The problem with superconducting media, however, is that they must be cooled to very low temperatures to operate. This increases the expense and power requirements for a system, in addition to reducing its reliability. The problem with the attenuating filter approach is that of bandwidth. It is very difficult, if not impossible, to design an attenuating filter which will maintain a constant group delay and desired attenuation characteristic over multiple octaves.
Accordingly, it would be highly desirable to provide a delay line in the form of an attenuating component which could be used in a bank of delay lines to provide a predetermined, constant time delay (i.e., phase delay with respect to frequency), and also which has a controlled loss (i.e., a loss which varies as a function of the frequency of the signal component passing therethrough) and a constant group delay. Such an attenuating circuit could be used to simulate the loss of a much longer delay line, while still providing a constant, shorter predetermined time delay.
The present invention is directed to an ultra wideband compensating attenuator intended for use as one delay line component in a plurality of banks of delay lines. The attenuator of the present invention provides a loss which can be matched to that of a different delay line having a much longer physical length, but which still provides a constant, much shorter time delay than the just-mentioned longer delay line. Thus, the attenuator of the present invention makes it possible to provide for equal loss through each one of a plurality of delay lines having different physical lengths, while still providing for shorter, yet constant time delay levels in accordance with the physical lengths of each of the attenuator components.
When the attenuator of the present invention is used in a circuit comprising at least one other delay line and a suitable switch for routing an input signal through either the delay line or the attenuator, the present invention makes it possible to provide for equal loss regardless of which path the input signal is routed. While this loss is still frequency dependent, the short time delay through the attenuator of the present invention provides exactly the same loss behavior as the longer delay line and maintains a nearly constant group delay.
The attenuator of the present invention is formed by placing a conventional (i.e., xe2x80x9cordinaryxe2x80x9d) microstrip transmission line in series with an engineered lossy microstrip line. While the conventional microstrip line has a group delay that increases with frequency, the engineered lossy microstrip line, conversely, has a group delay which decreases with frequency. When the two types of transmission lines are placed in series, the group delay changes can be made to effectively cancel each other over an extremely wide frequency range.
In one preferred form of the present invention, the attenuator comprises an engineered lossy line having a resistive material deposited along at least one longitudinal edge of a microstrip conductor to provide a predetermined degree of additional resistance to the conductor. In various preferred embodiments, this resistive material can be formed with a plurality of spaced apart, conductive metallic xe2x80x9ctracksxe2x80x9d to tailor (i.e., tune) the loss of the engineered lossy microstrip transmission line to achieve a desired degree of constant loss and/or constant time delay. The present invention thus makes it possible to duplicate a loss which increases with frequency, but does so over a much shorter physical length than a conventional delay line having a longer physical length.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.